The dynamic changes in microbial diversity during the aerobic composting of agricultural crop straw with additives were evaluated using high-throughput sequencing at four phases of composting (mesophilic, thermophilic, cooling and maturation phases).  In addition, the physicochemical parameters of the composting system were determined in this study.  The fermentation time of the thermophilic period was prolonged with the addition of urea or urea combined with a microbial agent.  The ratio of C/N and germination index variation indicated that the additives were favorable for composting, because the additives directly changed the physicochemical properties of the compost and had effects on the diversity and abundance of bacteria and fungi.  The abundance of operational taxonomic units (OTUs), diversity index (Shannon) and richness index (Chao1) of fungi and bacteria were found to significantly increase when urea+microbial agents  were added to straw in the thermophilic phase.  The relative abundance of the predominant bacteria and fungi at the phylum and genus levels differed during different composting phases.  The abundance of the phyla Firmicutes and Proteobacteria declined in the order of treatments SNW>SN>S (S is straw only compost; SN is straw+5 kg t^–1 urea compost; and SNW is straw+5 kg t^–1 urea+1 kg t^–1 microbial agent compost) in the thermophilic phase.  The abundance of the genera Staphylococcus, Bacillus and Thermobifida followed the same order in the mesophilic phase.  Ascomycota accounted for more than 92% of the total fungal sequences.  With the progression of the composting process, the abundance of Ascomycota decreased gradually.  The abundance of Ascomycota followed the order of S>SN>SNW during the thermophilic phase.  The abundance of Aspergillus accounted for 4–59% of the total abundance of fungi and increased during the first two sampling periods.  Aspergillus abundance followed the order of SNW>SN>S.  Additionally, principal component analysis (PCA) revealed that the community compositions in the straw and straw+urea treatments were similar, and that the bacterial communities in treatments S, SN and SNW in the mesophilic phase (at day 1) were different from those observed in three other phases (at days 5, 11, and 19, respectively), while the fungal communities showed only slight variations in their structure in response to changes in the composting process.  Canonical correlation analysis (CCA) and redundancy analysis (RDA) showed that total carbon (TC), NO₃^–-N (NN), electrical conductivity (EC) and pH were highly correlated with community composition.  Therefore, this study highlights that the additives are beneficial to straw composting and result in good quality compost.

 

Lead (Pb) in soil may accumulate in crops and enter the human body.  This study aimed to understand the speciation transformation and accumulation characteristics of exogenous Pb in calcareous soil with or without the application of soil amendments.  Field experiments with continuous maize cultivation have been carried out for two years.  The results showed that the contents of total Pb were slightly lower in 2016 than in 2015 for the same treatments; however, no significant difference between the years was observed.  Soil Pb existed mainly in the residual fraction without exogenous Pb addition, and its proportion was more than 33% of the total soil Pb in the control and Ca treatments.  When Pb was added to calcareous soil, Pb existed largely in the oxidizable and reducible fractions during the two-year experimental period, and those fractions made up over 83% of the total Pb.  The proportion of the water-soluble Pb, regardless of the addition of Pb, was the lowest and was less than 0.0019% in all treatments, but the addition of Ca and chitosan amendments reduced the water-soluble and exchangeable Pb contents.  The Pb content in different parts of maize followed the order root>leaf>stem>grain during the experimental period.  Although maize had low transfer and enrichment factors in calcareous soil, which make the Pb content in the grain show no significant difference among the five treatments in the same year, adding Ca and chitosan to calcareous soil can reduce the Pb contents of maize, especially reduce the Pb contents of root, stem and leaf.  Therefore, the addition of calcium and chitosan is an effective strategy for reducing Pb availability in calcareous soils.